The invention relates to a method of making a press felt, the method comprising forming at least one base fabric preform, and making the at least one base fabric preform into a closed-loop base fabric having predetermined dimensions in the cross machine direction and the machine direction of a paper machine and a first and a second longitudinal outer edges, and attaching at least one batt fibre layer onto the base fabric.
The invention also relates to a press felt comprising a closed-loop base fabric and at least one batt fibre layer attached onto the base fabric.
The aim of a press section in a paper machine is to drain a web as efficiently as possible without causing any deterioration in the quality of the web, however. From the viewpoint of energy consumption, it is most advantageous to remove as much water as possible already at the press section, whereby there is less need to dry the paper web at subsequent sections of the paper machine. Depending on the press structure, the press section of the paper machine employs a press felt on one or both sides of the web to be dried, into which press felt the water in the web is absorbed. The purpose of the press felt is to transport the water away after pressing without allowing it to migrate back into the web. In pressing, the paper web is conveyed on the felt to a gap between two rolls, i.e. so-called nip. Typically, there are one to four nips in succession, and in each nip water is pressed out of the web into the felt. Consequently, the felt should be such that the water can well be absorbed into the felt pressed in the roll nip. The press felts comprise a base fabric that provides the felt with a necessary space for water, for instance. To make the felt surface smooth, batt fibre is needled onto at least a base fabric surface facing the paper web. The batt fibre thus prevents markings from being produced on the web to be drained. In addition, by means of the batt fibre the water retention capacity of the felt can be adjusted to a desired level such that water is prevented from migrating back from the felt into the web, i.e. so-called rewetting is prevented.
Several alternative ways to form the base fabric are known. It is possible to weave the base fabric on a weaving machine directly into a seamless, endless loop of uniform quality. It is also possible to weave base fabrics provided with seam loops, and a desired number of base fabrics can be interconnected to form a closed loop. In that case the seam loops at the ends of the base fabrics are intermeshed and connected by inserting a seam yarn in a channel formed by the seam loops. The base fabric can be woven in flat or horseshoe form in known manners.
Irrespective of the structure and how the base fabric of the press felt is woven, known press felts have a drawback that longitudinal edge portions thereof act differently at the press section as compared with the midportion. It is possible for moisture to drain better at the edges than in the middle. Therefore, it is difficult to make the web to be dried homogeneous in the cross machine direction. Variation often occurs in the cross machine direction measuring profile of the web. It is possible to attempt to avoid the problem by adjusting the control parameters of the paper machine, but it is very complicated and demanding.
One more drawback of the known press felts is that the web does not accompany the press felt in a desired manner at the press section, or alternatively, the web does not always get detached from the felt in the desired manner. The reason is that at the press section the web adheres to the edge portions of the felt in a different manner as compared with the midportion. In general, the felt edges get clogged faster than the midportion, which changes so-called pick-up properties of the felt. Defective pick-up may cause cockle and other quality problems in the web to be dried, and deteriorate the runability of the paper machine.
The object of the present invention is to provide a novel and improved press felt for a paper machine, and a method of making the press felt.
The method of the invention is characterized by forming a base fabric of one or more base fabric preforms that are continuous in the transverse direction; providing both of the longitudinal edges of the press felt with longitudinal edge portions of a predetermined width from the longitudinal outer edge of the press felt and providing a midportion in the longitudinal direction of the press felt between the edge portions; and by arranging the structure of the press felt edge portions to be different from the midportion.
The press felt of the invention is further characterized in that the base fabric is continuous in the transverse direction of the press felt; that the press felt comprises longitudinal edge portions of a predetermined width in the transverse direction of the press felt and a midportion between said edge portions; and that the structure of the edge portions is different from the midportion.
The basic idea of the invention is forming a press felt, whose properties in the longitudinal edge portions of predetermined width differ in a pre-planned manner from the properties of the midportion between the edge portions. The edge portions of the press felt can be made to differ in e.g. permeance, grammage, surface properties, structure or other properties concerned, from the midportion of the felt. The invention has an advantage that running the paper machine is easier, because cross-machine direction variation in the moisture profile need not be compensated in a complicated manner by adjusting the control parameters of the paper machine as is the case with known press felts, in which the press felt is of substantially uniform quality throughout the press felt width. One more advantage is that the properties of the edge portions and the midportions of the felt can be arranged such that the felt pick-up takes place in a desired manner. The felt transfer is thus controlled in the press section, when the web adheres to and correspondingly becomes detached from the press felt as planned. Thus, it is possible to avoid quality problems associated with the web transfer. Moreover, the runability of the paper machine is better.
The basic idea of an embodiment of the invention is that the base fabric of the press felt comprises at least one base fabric preform that is substantially of the same width as the press felt and is woven such that yarn density at the base fabric edges is higher, or alternatively, lower than in the midportion. A change in the yarn density from higher to lower between the edge portions and the midportion can also be arranged gradually for a desired transition distance.
The basic idea of an embodiment of the invention is that the batt fibre attached to base fabric is employed to adjust the properties of the press felt in the cross-machine direction. The batt fibre layers to be attached can be denser or more open in the edge portions than in the midportion of the felt. Further, the number of batt fibre layers attached onto the midportion and edge portions of the felt can contribute to the press felt properties in the cross-machine direction. The solution has an advantage that it is possible to manufacture in advance standard base fabrics which can be customized with batt fibre to have desired properties in the cross-machine direction.
The basic idea of an embodiment of the invention is that the press felt is treated during manufacture with a suitable polymer material such that the polymer material penetrates into the felt partly clogging its structure. The treatment allows manufacture of a press felt with desired permeance in the cross-machine direction in the different portions. The solution has an advantage that it is possible to manufacture in advance press felts with predetermined properties, and not until the base fabric assembly and batt fibre attachment are the cross-machine direction properties of each press felt customized by means of the polymer material to be the desired ones.
The basic idea of an embodiment of the invention is that the press felt structure is provided, in desired cross-machine direction portions, with an extra, separate material layer, such as a polymer film provided with openings, by means of which material layer the permeance properties of the felt are adjusted to be desired in the cross-machine direction portions.